The majority of inks and toners used in the printing industry employ water insoluble polymers for print adhesion and durability. Water-based inks, such as used in ink-jet printing, can incorporate water insoluble polymer as dispersed particulates. The particulates are typically designed with a glass transition temperature (Tg) near room temperature to allow formation of a print-film on the printed substrate under normal ambient conditions. Alternatively, these water insoluble polymers can be coated on the surface of pigments in the form of polymer-encapsulated pigments.
Polymer encapsulated pigments of various kinds are known in the colorant art. The Sunsperse and Flexiverse polymer coated pigments from Sun Chemical Corp. are representative. While encapsulation methods and chemistries are known and well documented, the prior art has, here-to-fore, not taught polymer chemistries compatible or effective with thermal ink-jet. The prior art capsules are formed typically with dissolved polymers that are adsorbed onto the pigment surface. Inasmuch as the dissolved polymers are usually not crosslinked, the polymer surfaces of such known encapsulated pigments typically agglomerate under the high thermal shear and solvent conditions of the pen firing chamber, causing nozzle and ink channel blockages, or have excessive glass transition temperatures that prevent room temperature print film formation. Thus, typically, the incorporation of such polymer encapsulated pigments within thermal ink-jet inks either results in pen reliability or poor print durability colorant performance.
Conversely, the introduction of separate pigment and polymer particle components has several shortcomings as well. The combined surface areas of the latex and pigment particles adversely increase the viscosity of the ink-jet such that drop ejection rates and drop weight are limited to lower than otherwise norms. Such viscosities also negatively impact the collapse of nucleated vapor bubbles on thermal drop ejection, entrapping gas within firing chamber upon subsequent firings. Entrapped gas significantly retards ink-jettability. Likewise, differences between latex and pigment surface charges and charge levels can induce dispersion instabilities and performance changes within the ink over time. In addition, the pigment and latex often dissociate in the fibers of plain paper reducing both the otherwise durability and optical density of the print. Still further, many entrapped pigment particles are exposed above the surface of the latex film lowering print gloss and inducing gloss non-uniformities between different colored pigments within a print.
Thus, there is a continued need for development of pigment- and polymeric material-containing ink-jettable inks that are reliable when using thermal ink-jet architecture, and which have acceptable image quality and film forming properties.